Demand for hardwoods and their slow-growing nature have contributed to deforestation and produced related long-term ecological and environment problems. For example, tropical rain forests (a nursery for hard woods) are being stripped away at an annual rate of about 80,000 km2 to 100,000 km2. Such wholesale deforestation adversely affects plant and animal diversity and is believed to contribute about 20% of the world's greenhouse gas emissions. These environmental considerations mean that hardwoods are becoming more expensive.
Freshly felled timber has a considerable moisture content present as ‘free’ moisture within the cell cavities and ‘bound’ or ‘combined’ moisture saturating the cell walls. The freshly sawn lumber will typically lose about 50% of its total weight, shrink somewhat and become much stronger, harder and more durable during the subsequent seasoning [drying and stabilising] process. The seasoning process also improves timber workability and the bonding of adhesives and surface finishes. In the drying process the wood first loses the free moisture to reach the ‘fibre saturation point’ (FSP) where no moisture is contained in the cell cavities, but the cell walls are still saturated with bound moisture. The FSP occurs at 30-35% moisture content in hardwoods and 25-30% moisture content in softwoods. Timber does not shrink during drying until the FSP is reached then it begins to shrink at a roughly proportionate rate until equilibrium moisture content is attained.
A first form of wood modification the thermal was treatment of timber. In the “Feuchte, Warme and Druck” (FWD) process discussed by Burmester (circa 1973), there was controlled pyrolysis, i.e. degradation through the use of heat, of the wood at temperatures in excess of 180° C. While this heat treatment process provided an improvement in dimensional stability of between about 50% to 90%, bending strength was reduced by 30%. High temperatures induce strain in wood that can increase cracking or splitting.
As an alternative to pyrolysis, oil heat treatments have been carried out at temperatures greater than 180° C., but in low oxygen conditions. In the process executed by Menz Holz, the desired temperature was maintained for between 2 and 4 hours. Strength was reduced by ˜30%, although dimensional stability increased by ˜40%.
Chemical modification techniques include:                i) Etherification: Etherification is achieved from a reaction with carboxylic acids or acid anhydrides commonly acetylation and furfurylation;        ii) Dimethylol Dihydroxyl Ethylene Urea (“DMDHEU”) treatment;        iii) Reactive Oil Treatments; and        iv) Hydrophobation.        
In acetylation, impregnation is followed by heating to about 70° C. Acetic acid and unused anhydride have to be removed. Dimensional stability in the acetylated wood seemingly increases by ˜+75%, but there is only a moderate increase in hardness and strength is not influenced very much. Wood colour does not change in acetylation. However acetylated wood stains badly when exposed to the environment. Growth of mould is a particular problem.
Dimethylol Dihydroxyl Ethylene Urea (DMDHEU) can be used in the treatment of wood. However, use of DMDHEU produces volatiles including formaldehyde.
In reactive oil treatments, modified linseed oil effectively provides a maleic acid anhydride group. The process can uses modified existing creosote equipment.
The hydrophobation process changes the cell wall from hydrophilic to hydrophobic to improve the behaviour of the wood in wet conditions. There are a variety of chemical techniques, including the use of silanes, siloxanes and silicones, melamine and the so-called “Royal Process” in which curing is performed by a hot oil treatment and wherein biocides are added. The addition of any toxic component is undesirable because recycling of the wood requires that these toxic chemicals can be removed to prevent their entry into the environment.
Production of furan polymer modified wood has been described by M. Schneider, M. Westin and others. Water removal has been achieved by traditional kiln drying of the wood after the formation of the furan polymer in the wood tissue. Unfortunately elevated temperatures in any final drying step (as with other high temperature drying steps described in alternative processes) often induce a tensile strain in the modified wood, which strain leads to unacceptable cracking and deformation and, more generally, reduced quality. Avoiding such drying-induced strain is especially important in hardwoods, such as beech, ash and maple.
The diversity and generally enhanced properties produced by the chemical processes make their products more attractive. Specifically, the ability to replicate the properties of hardwoods using chemical processes is particularly advantageous when contrasted with those from the thermal and oil treatment processes. However, while some of these chemicals act for example to preserve the modified wood, they have detrimental effects. For example copper chromium arsenate proved to be a popular preservative, but chromium and arsenic are toxic. Also impregnating solutions that are prepared by chemical processes in complex resinmaking plants are expensive.
Furfurylation produces a modified wood having a high dimensional stability, high durability and high resistance to acid and alkali. Furfuryl alcohol (FFA) monomer is impregnated into the cells and subsequently polymerized by aqueous solutions. FFA is produced from hydrolysed biomass waste, e.g. molasses. FFA produces a highly branched cross-linked furan polymer grafted to the cell walls. Westin has explained the many ways that this molecule polymerises and of particular note is that FFA forms covalent bonds with lignin. Furfurylation marginally reduces the impact strength of the timber because the timber becomes harder and more brittle, but this is more than offset by the increased stiffness (of between 30% to 80%), increased dimensional stability (of between about 30% to 80%) and increased durability, i.e. resistance to insect and mould attack. Moreover, the impregnation of wood with furan-based polymers is advantageous in that the modified wood does not emit toxic substances either during the manufacturing phase or during the lifetime of the product.
For example, one of the processes employed to date to produce modified wood using FFA requires:                1. The furfuryl alcohol (FFA) to be mixed with water, catalyst and buffer and stored prior to use in a buffer tank. Since there has been a tendency for separation of the FFA from the water and the resultant mixture to have a short shelf-life, stabilisers e.g., borax have been added.        2. The wood to be placed in an autoclave, the mixture introduced and an over-pressure applied to impregnate the wood with the solution.        3. Then the wood is cured using a heating process. The application of heat causes the polymerisation to proceed at a meaningful pace. Unfortunately the greater the heat applied, the more detrimental is the effect on the wood. Also, different species of woods react differently to the application of high temperatures. Soft woods are better able to cope with aggressive heat treatment regimes. While reactants can be selected to effect curing at moderate temperatures as well as at high temperatures, reaction rates will differ accordingly.        4. After curing the wood is then dried in a kiln at which point any remaining volatiles and unreacted mixture are expelled to leave the wood dry and ready for the market. Following curing, the level of volatiles left in the wood is generally very low, i.e. less than 0.1% of the reactants, and these chemicals are generally qualified as being safe and suitable to remain within the modified wood.        
It is also known to react the base monomer FFA in two alternative ways: i) to produce a pre-polymer or oligomer; and ii) to add to the furan ring by producing a pre-polymer made by reacting furfuryl alcohol, formaldehyde and adipic acid. By reacting FFA with formaldehyde, a portion of the monomer is converted into bismethanofuran to produce a resultant mixture that is more reactive than pure FFA Maleic anhydride is added to the mixture as a polymerization catalyst before it is impregnated into the wood. Heat causes the mixture to polymerize in the wood. Curing is achieved at a temperature of between 70° C. and 200° C.
U.S. Pat. No. 2,947,648 discloses the use of FFA, maleic anhydride and water or alcohols as diluents, while sulphur dioxide is used as a penetrator/catalyst. An initiator must be added to the mixture. Gaseous sulfur dioxide catalyst is added to the product in a container after the mixture has been impregnated in a previous step. The mixture must have a low viscosity to penetrate wood. Water, methanol or ethanol can be used as a diluent. Uniformly treated, lumber-sized wood products are unlikely to result from this method because of limited gas penetration. The process also suffers from off-gasing of sulphur compounds and unreacted FA.
U.S. Pat. No. 2,909,450 discloses use of FFA, water and dibasic or tri basic organic acids. Water is used to dissolve a zinc chloride catalyst. While this mixture can be effective for thin timber samples, it does not easily penetrate lumber-sized samples. This leads to a multi-stage approach in which a ZnCl2 solution is firstly applied and allowed to dry, whereafter a second uncatalyzed FFA is applied to achieve enhanced distribution of reactant within the lumber. ZnCl2 has a high affinity for wood and therefore it is retained in the top layers of the impregnated material which, upon curing, leads to an egg-shell impregnation that leaves the core of the wood unprotected. Furthermore, a colour gradient often develops in the wood affecting the overall aesthetic appearance. Finally, unreacted FFA has been observed to leach from the wood over time, which off-gasing presents an odour problem and commercial loss from wasted FA. More critical is that ZnCl2 effects cellulose stability and therefore decreases the long-term strength of the modified wood.
WO 02/043933 discloses use of FFA, water and maleic acid as a catalyst.
U.S. Pat. No. 2,313,953 discloses the need for a buffer to stop the catalysts acting prematurely in a wood treatment process. Use of borax is disclosed.
U.S. Pat. No. 3,622,380 is restricted to the production of veneers. Atmospheric pressure soaking is used exclusively. The formulation is FFA, water, various metal salts to get different shades of color and a complexing agent.
Furfurylation techniques were developed by Schneider. In a first technique wood was impregnated with furfuryl alcohol and at least one other catalyst selected from maleic acid/anhydride, phtalic acid/anhydride, and stearic acid. These catalysts have similar affinity to wood as furfuryl alcohol and therefore penetrate wood at a similar rate. The impregnation solution was prepared by dissolving 5% to 20% of the catalyst in pure FA. Lower catalyst concentrations have longer storage life, but cure more slowly. Impregnation was carried out by a full-cell process. Wood samples were exposed to vacuum for 5 to 30 minutes and then a high pressure of 1 to 20 atmospheres for 20 to 60 minutes. The impregnated wood was then cured by stream, hot air, hot oil or high frequency (microwave) radiation in either: a) a one stage heating process at 140° C.; or b) a two stage heating process at 90° C. and then 140° C. Curing lasted for between 0.5 and 12 hours, with the 140° C. profile maintained for at least 1 hour to drive off uncured monomers and polymerization by-products. To avoid burning/charring of the wood, an oxygen-free atmosphere could be used.
In EP-B-1368167, Schneider diluted the FFA impregnation solution with water. The addition of water resulted in a two phase formulation and borax and sodium salts of lignosulfonic acids were added as stabilizers.
While heart wood has a better innate microbiological resistance due to its higher content of natural resins, these same resins inhibit the impregnation of the mixture into the heart wood. While FA can generally migrate into the heart wood, the migration of maleic acid or maleic anhydride is difficult. Consequently, FA may not be polymerized and it may leach out from the modified wood upon exposure to water.
In summary, most curing and drying methods in furfurylation techniques requires the use of elevated temperature that are generally detrimental to the overall treatment and conditioning of the wood. Elevated temperatures above about 140° C. and more particularly in excess of 150° C. alter the properties of the wood by damaging cell structures and by inducing internal strains that causes splitting or cracking of the wood; this is particularly true in the processing of hardwoods. Drying and curing are completed in successive and distinct steps in distinct chambers. Furthermore, acidic conditions, while considered necessary for polymerisation, affect shelf life of the mixture and can corrode reaction vessels. The use of volatile solvents, increases costs because of associated handling and re-cycling considerations.